Inspection apparatus



Jan. 22, 1952 W. ALTAR ETAL INSPECTION APPARATUS Filed March 27, 1947 'l'l'A'AvAn llllll l llllll AA AAA A v WITNESSES:

ZZ JZZW ATTORNEY Patented Jan. 22, 1 952 INSPECTION APPARATUS William Altar, Pittsburgh, and Otto J. M. Smith,

Scranton, Pa., assignors to Westinghouse Electrie Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application March 27, 1947, Serial No. 737,606

4 Claims.

by the X-ray absorption method.

It is well known that the absorption of monochromatic X-rays by a homogeneous material varies with the thickness of the material. The amount of absorption for a material of a given thickness depends upon the particular material used and the wave length of the X-rays. Consequently, it is possible to compare two articles by measuring the amount of X-ray absorption by the articles.

Comparison of twoarticles by measuring the X-ray absorption thereof may be usefully applied in a number of ways, such as in a thickness gauge for homogeneous materials. For example, it is desirable in connection with the rolling operations of a steel mill to measure the thickness of a substantially homogeneous steel strip without physical contact with the strip. The strip is moving at a very high speed, and an indication of the thickness as compared to a standard thickness is desired to permit correction and control of the rolling apparatus. To be useful, such an indication must, therefore, be given very quickly and accurately.

One of the familiar ways of measuring X-ray absorption is by means of a photographic film exposed to the X-rays after passage thereof through the material. Such an arrangement re? quires too much time to obtain the desired indication where rapid results are necessary as in a thickness gauge for a steel mill.

More recently, consideration has been given to the use of a photoelectric tube exposed to light from a fluorescent screen subjected to the X-rays after their passage through the material to be examined. However, prior arrangements in comparison apparatus employing photoelectric tubes have not been entirely-satisfactory for several reasons. Among these are the difiiculties arising from voltage fluctuations at the target of the X-ray tube, and from the instability of the photoelectric tube and its associated circuits.

It is accordingly an object of our invention to provide new and improved apparatus'for comparing two articles by the X-ray absorption method.

A further object of our invention is to provide novel apparatus employing a photoelectric tube for comparing two articles bythe X-ray absorption method.

Another object of our inventionisto provide new and improved apparatus for comparing the intensity of light from two different sources.

A still further object of our invention is to provide new and improved apparatus including photoelectric means for comparing the intensity of light from two different sources.

Still another object of our invention is to provide a novel arrangement for improving the stability of a photoelectric tube and its associated circuits.

More specifically, it is an object of'our invention to provide novel apparatus employing a photoelectric tube for comparing two articles by the X-ray absorption method in which disturbing effects of voltage fluctuations at the target of the X-ray tube and of instability of the photoelectric tube and its associated'circuit's are minimized or avoided.

In accordance with our invention, as applied to comparison apparatus, X-rays from a singlesource are projected through the two articles to be compared. A fluorescent screen is associated with each article-to receive the X-raysafter passage thereof through the corresponding article. Light from the two screens is projected into a single photoelectric tube periodically but in opposite phase relation. Thus, if the energization of the photoelectric tube by the two light beams is unequal, an alternating output is obtained from the tube, the amplitude of which is a measure of the difference in X-ray absorption by the two articles. The alternating output of the photoelectric tube may be amplified by an alternatingcurrent amplifier and eventually applied to an indicating device, such as a meter or relay.

Since the intensities of the X-rays through both articles change only in constant proportions under voltage or current fluctuations in the sinle X-ray tube, the relative responses in the output of the single photoelectric tube for the two articles is not affected substantially by such fiuctuations. Moreover, the eii'ect of variations in the sensitivity of the photoelectric tube are reduced since such variations would afiect both responses equally.

To obtain complete compensation for variations in the sensitivity of the photoelectric tube, as may be brought on by fatigue, a compensating circuit may be provided. This compensating circuit, in effect, compares the total voltage output of the photoelectric tube with a standard and varies the voltage supplied to the photoelectric tube in accordance therewith, thereby compensating for variations in tube sensitivity.

..'I'l1e featuresoi; our. invention, which we contured disc employed in the apparatus of Fig. 1. As shown in Fig. 1, X-rays are generated by means of an X-ray tube 3 energized from a source of alternating current fi-through a transformer and the usual full wave rectifier 9. The X-ray tube 3 is positioned substantially midway between a metal strip H of a standard' thickness hereinafter referred to as the. standard strip, and a metal strip l3 of the same material of an unknown thickness hereinafter referred to as. the test strip. The arrangement illustrated corresponds to a thickness gauge, as applied to a rolling mill in which the standard sheet II is held in a. fixed position while the test strip i3 is moved rapidly past the X-ray tube in a direction perpendicular to the, plane of the drawing. The X-ray tube 3 is adapted to direct substantially these-me quantity of X-rays onto each of the strips II and i3.

.Afiuorescent screen I5 is positioned on the opposite side of the standard strip H from the X-ray tube 3 and is arranged to receive X-rays after passage thereof through the standard strip, The screen is of a suitable material, such as silver activated zinc sulphide or calcium tun state, which converts incident X-radiation into light of a proportional intensity.

A similar screen I! of the same material and size is positioned on the opposite side of the test strip I3 from the X-ray tube 3 in the same relative position as the first screen [5 bears to the standard strip ll. Screen I1 is arranged to receive X-rays after passage thereof through the test strip l3.

The light from. the two screens land I! is to be projected onto a single photoelectric tube I9 periodically but in substantially opposite phase relation. This may be accomplished by any one of several suitable arrangements. We prefer to have each screen l5 and IT in the form of a coating on the end of a rod 2| and 23, re-, spectively, of transparent thermoplastic resin, such as the resin known as Lucite, which is capable of conducting light lengthwise thereof.

with but a small loss of light in other directions.

Thetwo light conducting rods 2| and 23 are shaped and positioned so that the ends thereof opposite from the screens l5 and I! are adapted to. direct light from the screens toward the light sensitive area of the photoelectric tube I9. The two ends 25 and 21 of the light conducting rods 2i and 23, respectively, opposite from the screens l5 and I! are positioned side-by-side, as shown in Fig. 2, and have substantially the same crosssectional area and shape.v

Interposed between the ends 25 and 21 of the light conducting rods 2! and 23 and the photoelectric tube IQ is an apertured disc 29-. The disc is arranged to be rotated in synchronism with the'supply voltage for the X-ray tube 3 by a synchronous motor 3| energized from the alter nating-current source 5, A plurality of apertures 33 is provided in the disc 29, the apertures SS-being located, so that each aperture ismoved .t.ric tube I9 while the disc is rotating.

in front of the ends of the two rods successively during each revolution of the disc. Preferably, each aperture 33 is the same size and shape as the end of one of the light conducting rods 2| and 23 with the space between adjacent apertures being of just suflicient size to completely cover the end of a red. It follows that light from one or the other or both of the rods 2! and 23 is always directed onto the photoelec- Moreover, the sum of the cross-sectional areas of the ends 25 and 21 of the rods 2| and 23, which are uncovered at. any time, remains substantially constant.v Therefore, if the light available at the .end of the rod 23 associated with the test -stripl3 isequal to the light available at the end of the rod 2| associated with the standard strip llfthedightprojected onto the photoelectric '1 tube-i9 remains substantially constant.

On the other hand, if the lights from the two rods 2! and 23 areunequal, analternating light stimulus is provided for the photoelectric tube l9.

The photoelectric tube I9 is preferably of the secondary emission multiplier type, known as a photomultiplier tube, suchas an RCA 931-A tube. The photoelectric tube 12; includes an anode 3'! and a cathode 35, as well as nine dynodes 4!, 42, 43, 44, 45, 46, 41, 48 and interposed between the cathode 35 and the anode 37 to provide the multiplier effect. Supply voltages for the photoelectric tube [9 are obtained from a pair of series-connected substantially constant direct-current voltage sources 5i and 53. A voltage divider 55 is connected across the series-connected voltage sources 5! and 53 with its positive terminal connected to ground and its negative terminal connected to the cathode 35 of the photoelectric tube 19. Intermediate taps 5E, 62, S3, 64, G5, 86, ET, GBand E9 are provided on the divider 55 and are spaced therea long between the negative and positive terminals with the dynodes of the photoelectric tube is connected to the intermediate taps in their order of location between the cathode 35 and the anode 3?, the ninth dynode 49, which is closest to the anode tfland furthest from the cathode 35, being connected to. the most positive or the intermediate taps 69 while the first dynode ii, closest to the cathode 35, is connected to the most negative intermediate tap 6|. A capacitor H is connected between the eighth and ninth dynodes 43 and 49 for purposes which will be more fully explained in connection with the compensation circuit.

, The anode 31 of the photoelectric tube is is connected through a resistor 13 to the ground. Thus, when light is directed onto the photoelectrio tube 19, current fiows through the resistor. 13 creating a voltage thereacross. When the light isfsubstantially constant, as is provided when the X-ray absorption by each of the two strips H and I3 is the same, the voltage across theresistor i3 is substantially constant. However, 'when the X-ray absorption of the two strips is unequal, an alternating-voltage component appears across the resistor 73.

"An alternating-current amplifier 15 is provided to amplify "the alternating component which appears across the resistor l3 in the anode circuit of the photoelectric tube iii. The amplifier 15 includes a vacuum tube 11 having a grounded cathode 19 and a control electrode 81 connected through -.a blocking capacitor 83 to the ungrounded end of the resistor '53. The anode-riot the amplifier 'tube- 1-1 is connected through a high-Q resonant circuit, comprising an inductor 81 and a capacitor 89 tuned to the desired frequency, and a substantially constant direct-current voltage source 9| to the ground. The screen grid 93 of the amplifier tube is connected to the junction point between the voltage source 9! and the resonant circuit.

The output of the amplifier 15 is applied to an indicator. In the event a mere indication of the existence of a difference in X-ray absorption of the two strips II and I3 is all that is desired, the indicator may be a simple indicating device, such as a meter or relay. However, if it is desired to indicate which of the two strips II and I3 has a greater X-ray absorption and the amount of the difference, a more complicated indicator may be provided, as illustrated. This indicator includes an algebraic rectifier 95 connected to the amplifier 15 to convert the output thereof into a direct current of a magnitude which is a measure of the difference in X-ray absorption and a polarity dependent upon which of the strips has the greater absorption. A suitable polarized direct-current meter 91 is provided in the output circuit of the algebraic rectifier.

, The algebraic rectifier 95 is connected to the anode 85 of the amplifier tube 11 through a blocking capacitor 99. The algebraic rectifier includes two pairs of vacuum tubes, one pair of vacuum tubes IN and I03 being inversely connected between the blocking capacitor 99 and one end terminal I05 of the meter 91 while the other pair of vacuum tubes I01 and I09 is inversely connected between the blocking capacitor 99 and the other end terminal I I I of the meter 91. The center terminal II3 of the meter 91 is connected to ground.

The control circuit of each of the vacuum tubes I9I, I03, I01, I09 includes a biasing voltage source I I5 and an individual secondary winding II1 of a a transformer H9. The arrangement is such that a voltage pulse of one polarity supplied through the transformer II9 causes the control grids IZI and I23 of both tubes MI and I03 of one pair to become positive with respect to their cathodes I24jand I26, while a voltage pulse of opposite polarity supplied through the transformer II9 causes the control grids I25 and I21 of the other, pair of vacuum tubes I01 and I09 to become positive with respect to their cathodes I28 and It is-apparent that the alternating-voltage component across the resistor 13 in the anode circuit of the photoelectric tube I9, when the standard strip II has a greater absorption than the test strip I3, is approximately 180 out of phase with the alternating-voltage component when the absorption of the standard strip I I is less than that of the test strip I3. It follows that at any given instant of time, other than one in which the alternating component is zero, the terminal of the algebraic rectifier 95 connected to the blocking capacitor 99 is positive with respect to the ground if a particular one of the strips has the greater absorption but is negative with respect to ground if that one particular strip does not have the greater absorption. Then, if the terminalconnected to the blocking capacitor is positive with respect to ground at the instant the right-hand pair of tubes IM and I03 in the algebraic rectifier have positive control grids, current flows through the upper tube IOI of the pair and through the meter 91 to ground. If the'control grids of the left-hand pair I01 and I09 are then made posi-' tive' in the next half period ofthe alternating component, the terminal of the algebraic rectifier connected to the blocking condenser 99 becomes negative with respect to ground and current flows from ground through the meter 91 and the lower tubev I99 of. the left-hand pair. As a result, the current. through the meter 91 is in the same direction in both half periods of the alternating component to cause the indicating arm of the meter to move in one direction from its zero position. The extent of movement of the indicating arm depends upon the magnitude of ;the currentin the meter 91 and is a measure of the difference in X-ray absorption. 4 e

If, at the instant the control grids of the righthand pair of tubes IOI; and I03in the algebraic rectifier 95 are positive, the terminal connected to the blocking capacitor 99 is negative with respect to ground, current flows from the ground through the meter 91 and the lower tube I03 of the pair. Similarly, if the grids of the left-hand pair I01 and I09 are made positive during the next half period of the alternating component, current flows from the terminal connected to the blocking capacitor 99 through the upper tube I01 2 of the pair and the meter 91 to be grounded.

As the result, the indicating arm of the meter 91 is moved from its zero position in the opposite direction from that in the situation previously described.

The primary winding I29 of the transformer IE9 supplying the control circuits of the vacuum tubes IOI, I03, I01 and I09 in the algebraic rectiher 95 is connected to be energized at intervals determined by a suitable phasin apparatus. We

" prefer to control the phasing operation by means of a commutator I3I mounted on the rotating disc 29. A pair of brushes I33 and I35 engage the surface of the commutator I3 I and, as shown in Fig. 2, are positioned to be electrically interconnected by the commutator just slightly prior to the time at which the end 25 of the light conducting rod 2I associated with the standard strip I I is completely uncovered. It is to be noted that the angular brush width and the width of each conducting segment I36 on the commutator is one-half the angular width of an aperture. Then, the disc must move angularly an amount equal to two widths of a brush between interconnecting and disconnecting the brushes. As a result, the commutator I3I disconnects the brushes I33 and I35 just slightly prior to the time the end 21 of the light conducting rod 23 associated with the test strip I3 is completely uncovered.

Externally, one of the brushes I33 is connected to the negative terminal of a substantially constant direct-current voltage source I31 while the other brush I35 is connected to ground and to the positive terminal of the source I31 through a resistor I39. Thus, an approximately square wave voltage is produced across the resistor I39 as the disc 29 rotates. A capacitor MI and a second resistor I43 are connected in series with each other and in parallel with the first resistor I39, the capacitor I4I being connected to the junction point between the first resistor I39 and by the-commutator -I3I and each such pulse lasts for. a short period during which the end 25 of the rod 2| associated with the standard strip H is completely uncovered. The pulses of the other polarity occur each time the brushes I33 and its are disconnected by the commutator and each such pul'se lasts for a short period during which the end 21 of the rod 23 associated with the test strip i3 is completely uncovered Since the secondary windings of transformer H9 are wound alike, the control grids of one pair of vacuum tubes ofthe algebraic rectifier are made positive by pulses of one polarity while the grids of the other pair are made positive by pulses of opposite polarity. The characteristics of the tubes I9I, I83, I91 and I99 and the relative volta es are such that the currentconducted through one or. the tubes when its grid is made positive is approximately proportioned to the magnitude of the anode voltage.

' A compensation circuit I41 is also arranged to'be operated in phase with the disc 29 through the pulses developed across the second resistor M3 in the commutator circuit. The compensation circuit I4! includes apair of vacuum tubes I99 and IEI, preferably tetrode tubes, the oathodes l53 and I55 of which are connected to ether to the negative terminal of another substantially constant direct-current voltage source I51 and to ground. The anode I59 of the first tetrode I99 is connected through a resistor I iii to the positive terminal of the voltage source I51. The anode I93 ofthe second tetrode I5I is also connected to the positive terminal of the voltage source I57 through another resistor I655. The control electrodes I 61 and I69 of the two tetrodes I99 and E51, respectively, are connected to the grounded cathodes E53 and I55 thereof through individual'resistors- Ill and I13, respectively, and a biasing voltage source I15. The control grid I99 of the second tetrode I5I is also connected to the anode 3? of the photoelectric tube I9. The screen grids IT! and I19 of the two tetrodes I99 and IEI, which are also employed in controlling conduction therethrough, are connected to the junction oint I45 between the capacitor I4! and second resistor I49 in the commutator circuit.

The biasing voltage for thecontrol grids I5; and 599 of the two tetrodes I 49 and i5! remains substantially constant and is of sufficient magnitude to-prevent the 'tetrodes from conducting except during a period when a positive voltage pulse is applied to the screen grids Ill and H9. Since thecontrolgrid I69 of the second tetrode i5l isconnected to the anode 31 of the photoelectric tube I9, the output voltage of the photoelectric tube as appears across resistor I3 is also impressed in the control circuit of the second tetrode. When the commutator I SI causes a voltage pulse making the screen grids I I1 and 119 of the tetrodes'positive, current flows through both tetrodest However, because of the negative voltage applied inthe control circuit of the second tetrode. I9! from the output of the photoelectric tube lacurrent through the first tetrode I 49 is greater than that through the second tetrode I'5I. As a result, the anode 159 of the first tetrode I49 is at that time more negative than the anode IE3, of the second tetrode I5I.

-An auxiliary amplifier tube I8! is provided to amplify the. difierence between the potentials of the anodes I59 and I63 of the two tetrodes I49 and I5I. The cathode I83 of. the auxiliary tube IBI is connected to theanode I63 ofthe second iredalflgwh le. the c n 9 t I85 is con:

2! associated with the standard strip II.

nected to the anode I59 of the first tetrode I49. The anode I81 of the auxiliary tube I8I is connected through a resistor I89 to the positive terminal of the voltage source I51 for the two tetrodes.

Another vacuum tube I9! has its anode I93 connected to the ninth dynode 49 of the photoelectric tube I9 while the cathode I 95 of the vacuum tube I9I is connected to the eighth dynode 48. The control grid I91 of the vacuum tube I9! is connected to the anode I81 of the auxiliary tube I8! and is also connected to the cathode I95 of the vacuum tube i9I through a resistor I99 and a biasing voltage source 2III.

When the anode I59 of the first tetrode I49 becomes more negative than the anode I63 of the second tetrode I5l, the control grid I85 0! the auxiliary tube I9I becomes negative with respect to the cathode I83 thereof. This results in a positive voltage pulse on the control grid is? of-the vacuum tube I9I, which grid I9'I is normally held at a high negative voltage by the biasing voltage source 29L While the grid I9? is negative, tube I9I is nonconductive and the condenser II between the eighth and ninth dynodes 49 and 99 is charged from the second voltage supply source 53. However, when the positive voltage pulse appears on the control grid #92, the tube I9I becomes conductive and discharges the condenser II to an extent determined by the amplitude of the positive voltage pulse on the grid 91. The amplitude of this pulse, of course, is determined primarily by the magnitude of the output voltage of the photoelectric tube I9.

Since the gain of the photoelectric tube I9 is determined by the voltage between the eighth and ninth dynodes d8 and 49, a discharge of the condenser it reduces the gain. In between successive positive pulses from the commutator, the condenser ll gradually recharges. It is then apparent that the gain of the photoelectric tube I9 is reduced until the output voltage of the photoelectric tube 59 is just sufficient to maintain the voltage of condenser H substantially constant. Should the sensitivity of the photoelectric tube I9 vary, such variation is immediately reflected in the output voltage of the photoelectric tube and results in a change in the voltage of the condenser H to vary the gain of the photoelectric tube and thereby compensate for the change in sensitivity.

It is to be noted that the voltage pulse from the commutator MI is phased so that the tetrodes I49 and ISI may conduct only during an interval when the output voltage of the photoelectric tube I9 is substantially the result of energization of the photoelectric tube I9 by light from the rod Thus, in effect, the compensation circuit compares the output voltage at that interval with a standard voltage to give an accurate indication of the sensitivity of the photoelectric tube I9.

Although we have shown and described the apertures 33 in the disc 29 as being uniformly spaced to provide a substantially constant quantity of light on the photoelectric tube i9 so long. as the absorption of the standard and test strips II and I3 are equal, it obviously is not necessary that such an arrangement be employed. Any suitable arrangement in which light from the two rods H and 23 is directed onto the photoelectrictube I9 periodically, but in substantially opposite phase relation, is satisfactory.

. When. theX-ravtube 3 is supplied. with a periodically pulsating voltage, as in the arrangement illustrated, it is preferable that thenumber of apertures 33 in the disc 29 be selected so that the frequency at which light from one of the light conducting rods is directed'onto the photoelectric tube is n times the frequency of the supply voltage for the X-ray tube, where n is other than a whole number. Otherwise, the photoelectric tube may be exposed to light from one rod at the same point always on the periodic pulsation of the supply voltage for the X-ray tube and thereby a systematic error may be introduced. Thus, if a (SO-cycles per second alterhating-voltage source is employed, the pulsating supply voltage on the X-ray tube 3 beats 120 times a second and the frequency of the supply voltage is either 120 cycles per second or 60 cycles per second depending upon whether or not the half periods of the source voltage of opposite polarity are identical. The disc 29 is then to be rotated in synchronism with the source voltage, for example, at the rate of 1800 revolutions per minute. The preferred number of apertures in the disc is then an odd number, for example, 3 l, in which case the frequency of the light from one rod is 930 exposures per second or 15.5 times 60 cycles per second or 7.75 times 120 cycles per second. In this manner any systematic errors resulting from a systematic variation in the X-ray yield because of the systematic variation in the supply voltage is avoided.

It is also possible by spacing the apertures 33 other than uniformly to change, in effect, the quality of the X-radiation so far as the photoelectric tube response is concerned. For instance, if a certain application requires a narrow X-ray spectrum, the apertures 33 may be arranged in groups so that the photoelectric tube I9 is exposed to light only when the target voltage of the X-ray tube 3 is at or near its positive maximum.

While we have shown and described a specific embodiment of our invention, we are aware that many modifications thereof may be made without departing from the spirit of the invention. It is not our intention therefore to limit our invention to the particular arrangement shown and described.

We claim as our invention:

1. Apparatus for comparing two articles comprising a source of X-rays adapted to project X-rays on to each of said articles individually, a screen associated with each of said articles and positioned to receive X-rays from said source after passage thereof through the corresponding article, said screens being of a material which converts incident X-radiation into light of a proportional intensity, a photoelectric device having an output circuit associated therewith, means for directing light from said screens on to said photoelectric device periodically so that said screens begin to illuminate said photoelectric device alternately, an amplifier connected to said output circuit to amplify an alternating current therein resulting from unequal light from said screens, an algebraic rectifier operated in phase with said light directing means and connected to said amplifier to convert the output thereof to a direct current of a polarity determined by which of the screens provides the greater light, the magnitude of said direct current being a measure of the difference of said lights, and a polarized current responsive device connected to said rectifier to be energized by said direct current.

dividually, a screen associated with each of said articles and positioned to receive X-rays from said source after passage thereof through the corresponding article, said screens being of a inaterial which converts incident X-radiation into 1 light of a proportional intensity, a photoelectric device having an output circuit associated therewith, means for directing light from said screens on to said photoelectric device periodically but in substantially opposite phase relation, an amplifier connected to said output circuit to amplify an alternating current therein resulting from un equal light on said screens, an algebraic rectifier operated in phase with said light directing'means and connected to said amplifier to convert the 29 output thereof to a direct current of a polarity determined by which of the screens provides the greater light, the magnitude of said direct current being a measure of the difference of said light, and a polarized current responsive device con- 5 nected to said rectifier to be energized by said direct current.

3. Apparatus for comparing two articles comprising a source of X-rays adapted to project X-rays onto each of said articles individually, a

30 screen associated with each of said articles and positioned to receive X-rays from said source after passage thereof through the corresponding article, said screens being of a material which converts incident X-radiation into light of a proportionai intensity, a photoelectric device having an output circuit associated therewith, circuit means connected to said photoelectric device to provide an output circuit therefor and includin means providing a supply voltage, the magnitude of t which determines the magnitude of output voltage in said output circuit for a given light on said photoelectric device, and a capacitor circuit responsive to said output voltage to vary the magnitude of said supply voltage to said photoelectric 5 device in such a way as to compensate for fluctuations in the sensitivity of said tube, means for directing light from said screens onto said photoelectric device periodically so that said screens begin to illuminate said photoelectric device, an

59 amplifier connected to said output circuit to amplify an alternating current therein resulting from unequal light from said screens, an algebraic rectifier operated in phase with said light-directing means and connected to said amplifier to convert the output thereof to a direct current of a polarity determined by which of the screens provides the greater light, the magnitude of said direct current being a measure of the difference of said lights, and a polarized current responsive device con- 39 nected to said rectifier to be energized by said direct current.

4. Apparatus for comparing two articles comprising a source of X-rays adapted to project X rays onto each of said articles individually, a

screen associated with each of said articles and positioned to receive X-rays from said source after passage thereof through the corresponding article, said screens being of a material which converts incident X-radiation into light of a pro- 7() pcrtional intensity, a photoelectric device having ,an output circuit associated therewith, said output circuit including means providing a supply voltage, the magnitude of which determines the magnitude of output voltage in said output circuit for a given light on said photoelectric device,

ii means operated in phase with said light-directing means for comparing the output voltage with a standard voltage during an interval when the output voltage results substantially from light from a preselected one of said articles, and means responsive to said comparing means for varying the magnitude of said supply voltage, means for directing light from said screens onto said photoelectric device periodically so that, said screens begin to illuminate said photoelectric device, an

amplifier connected to said output circuit to am- REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,806,197 Hardy et a1 May 19, 1931 2,076,553 Drinker et a1 Apr. 13, 1937 2,198,233 Snyder Apr. 23, 1940 2,225,439 Arens et a1 Dec. 17, 1940 2,382,251 Parket et al Aug. 14, 1945 2,457,747 Sweet Dec. 28, 1948 2,469,206 Rich May 3, 1949 2,467,844 Michel Apr. 19, 1949 OTHER REFERENCES Smith: General Elec. Review, March 1945, pages 13-17.

Michel and Rich: General Elec. Review, February 1947, pages 45-48.

Moriarty: General Elec. Review, February 1947, pages 39-42. 

